Well it is backed by a dollar...worth of HIVE. The dollar is a unit of account, not the currency or a T-bill like Tether or USDC.

What happens if a big corporation buys up a lot of Hive, converts it to HBD, and stakes it for the 20 percent? How will Hive and HBD prices react to that?

Supply and demand tells me the price will shoot up. The buying off the open market would send prices higher, means less HIVE creates more HBD.

The same is true if more HBD is required for transactions. That is why every project on Hive that accepts payment should use HBD. It is the medium of exchange and, as transactions grow, requires more units. That can really only come, in a large amount, from HIVE conversion.